Under certain circumstances, a telephone operating company may provide telephone services for an RD(ringdown) subscriber, wherein the RD subscriber is a subscriber having an RD telephone set. The RD telephone set is an obsolete-type telephone set, being neither a dial nor a push-button type of telephone, and for an RD subscriber to make a call to another subscriber, he just off-hooks a hand set or revolves a knob attached to the RD telephone. In response to the off-hooking or the revolving of the knob, an ESS(electronic switching system) which links itself to the RD telephone set informs the incoming of a signal to an OPS(operator position system) which is connected to the ESS, the ESS being located in a CO (central office), wherein the CO is an office of a telephone operating company where the ESS, transmission system, etc. are deployed to provide various telephone services. The OPS is an equipment for providing a variety of telephone services as well as establishing a call connection in response to a call request of the RD subscriber to interface with the ESS.
An operator at the OPS talks on the line with a calling RD subscriber, or the caller, to get the information on the call, e.g., the telephone number to be called, and the like. Then, the operator establishes a call connection between the caller and another subscriber the caller wants to be connected to by using the telephone number given by the caller.
In addition to the establishing of a call connection between subscribers, there are other functions that the OPS performs. One of such functions is related to a tariff.
An automated tariff system automatically performs tariffing services. By using the automated tariff system, a lapse time, i.e., the duration between a connection establishment and a connection release between the caller and the receiver, and the classification of a caller, e.g., a restriction to a subscriber and a grade of a subscriber, are automatically determined by using the information pre-stored therein.
On the other hand, a tariff on the RD subscriber may be performed by a semi-automatic method, wherein the lapse time is decided and manually inputted to the ESS by an operator at the OPS for the purpose of the tariffing services.
In order to decide the lapse time, the operator at the OPS needs to wiretap from the beginning of an on-the-line talk through the end or monitor periodically to check if the current caller is still in off-hook state. Such wiretapping imposes a great burden on the operator; and also is costly.
Although periodic monitoring is regarded more efficient, no equipment designed to periodically monitor an on-the-line talk has been developed yet.